In the operation of rotary drilling an oil or gas well, drilling fluid having a prescribed density is used during the drilling operation for several purposes including to balance the formation pressure which increases as the depth of the hole increases. Such drilling fluid or "mud" is pumped down the drill string, through the bit, and returned to the surface in the annulus between the drill pipe and the borehole wall. This process is known as "circulation" of the drilling fluid. If the density of the drilling fluid is excessive this can result in "breaking down" the formations encountered by the bit with the resultant loss of fluid into such broken down formations. This sort of condition results in the loss of the fluid communication path and a formation pressure overbalance, and is known as "lost circulation". Lost circulation can result in extensive well damage and is, altogether, an undesirable and possibly dangerous condition which must be avoided.
This condition may generally be avoided by appropriate selection of the density of the drilling fluid used. The fluid density of the drilling fluid or mud is usually controlled by the addition of heavy earth materials, such as barite, in known ratio to the fluid volume to produce a controlled, known density, mud.
When the well has "bottomed out" and is to be completed for production, it is necessary to set steel casing into the borehole to line the walls thereof in order to prevent caving in of the sidewalls. The steel casing, of course, must fit inside the hole diameter. This leaves an annulus between the outer surface of the casing and the borehole wall. This annulus is filled with oil well cement, having certain desired properties, in order to prevent fluid communication along the casing/borehole annulus. Such undesired fluid communication can result in well damage and loss of commercial production potential. The oil well cement is placed in the annulus between the casing and borehole wall by pumping a highly fluid cement slurry down a string of production tubing and out, either the casing bottom, or perforations in the casing and into the casing/borehole annulus. The volume of cement necessary to fill the annulus may be calculated since the outer diameter of the casing and the borehole diameter are known. Again, however, the density of such cement fluid slurries cannot exceed certain limits or the "lost circulation" condition will be encountered. Thus it is necessary to be able to control the density of the oil well cement used in cementing operations, just as for the drilling fluid during the drilling operation.
Mixtures of certain proportions of water with well cements are called "normal density" slurries, where the slurry and the set cement have about optimum properties for pumping into wells. For example, the normal density of American Petroleum Institute (API) Class H cement slurry is 16.4 pounds per gallon (ppg). Normal density of Class G slurry 15.8 ppg and Class C slurry is 14.8 ppg. It is often necessary to use "extended slurries" of cement having lower density than a "normal" in order to prevent lost circulation due to excessive hydrostatic pressure when the slurry is pumped into the well. Extended slurries having densities of 12 ppg, or less, are routinely used. In fact most cement slurries used in the industry are extended slurries.
Most common low density, or extended, slurries are made by mixing excess water, compared to the amount for normal density slurries and additives, such as bentonite. The bentonite, a form of ground up high density clay, is used to prevent settling of the cement particles unduly, before the cement sets, or solidifies. Cement settling prior to setting is usually evaluated in the laboratory by the API free water test. In this test, the volume of free water which accumulates on the top of the cement slurry in a specified diameter 250 cubic centimeter (cc) graduated cylinder after setting for 2 hours at room temperature is determined. Current practice is to require that the free water be below some maximum amount, for example 5 cubic centimeter (cc).